Coding is the name of the process of writing the programming code required for a game to run. Things like the rules of CTF, the way a weapon fires when u fire it (lol) and the game physics are all controlled/created using coding.

To "do" coding, all you really need is a copy of notepad, a compiler and a sound knowledge of the C++ 3GL programming language. There are many resources online where you can learn to code and if you're interested in taking it up, then I wish you good luck as it can be quite hard some times.